Virtual Instrumentation

Published on Aug 15, 2016

Abstract

A virtual instrument in principle is a computer based software driven instrument for test, measurement or process control applications. Virtual instruments are thus composed of layers of software and hardware having a virtual control panel that only appears on a computer display. In virtual instrumentation systems the appearance of real instruments and measurement workbenches are mimicked so that they resemble the interfaces of their real counterparts.

Instead of conventional text-based procedural programming languages where syntax and punctuations are sensitive aspects of a program, visual or graphics-based programming is ideal for virtual instrumentation. Different DAQ (Data Acquisition) Cards are available which are interfaced with our PC. Data Acquisition devices combine data acquisition with the processing power of a computer.

With DAQ devices, the hardware converts the incoming signal into a digital signal that is sent to the computer. The computer receives raw data. Software takes the raw data and presents it in a form the user can understand. Software manipulates the data so it can appear in a graph or chart or in a file for report. The software also controls the DAQ system, telling the DAQ device when to acquire data, as well as from which channels to acquire data.

A virtual instrument in principle is a computer based software driven instrument for test, measurement or process control applications. Virtual instruments are thus composed of layers of software and hardware having a virtual control panel that only appears on a computer display. The use of computer allows for confining the hardware components dedicated to the measurement application to the data input/output subsystems responsible for data acquisition and generation.

All operations of the test and measurement procedure like control of information, display and analysis of acquired data, data management such as archiving, printing, Internet publishing etc are performed in the software. Thus a virtual instrument defines its specific function through software programming and it is software intensive.

The major advantage of virtual instruments as compared to traditional hardware based instruments is its flexibility. The functionality and user-machine interface of conventional test instruments are manufacture- defined and end-users have no control to expand or modify the existing functions. In comparison, the functionality of a virtual instrument can be explicitly defined, modified and expanded by its users through software programming. Once the platform of a virtual instrument is established, users can define specific test functions they need, adopt a user interface they like and modify the instrument functionality, whenever it is necessary.

The computer based measurement systems, even if effective and efficient from the measurement point of view, may appear very different from the conventional instruments as far as human user interface is concerned. In virtual instrumentation systems the appearance of real instruments and measurement workbenches are mimicked so that they resemble the interfaces of their real counterparts .

The advanced graphics features of most of the modern personal computers allow for implementing a realistic user interface of the computer based instruments and workbenches, very similar to the real ones. The front panel presented on the computer screen comprises the graphic representation of conventional controls like knobs, switches, selectors, potentiometers and indicators like graphs, lights etc. and provides the interface for interactive operation.

Graphic flexibility also allows for easy merging of the front panels of several instruments to create a comprehensive workbench of virtual instruments with all the components of the envisioned application. This simplifies the user interaction with monitoring and control system since it concentrates all relevant instruments in one single inter-related display discarding the information and functions that are not of direct and immediate interest.

The effectiveness and efficiency of virtual instrument design is largely due to the user-friendliness associated with the programming process itself. Instead of conventional text-based procedural programming languages where syntax and punctuations are sensitive aspects of a program, visual or graphics-based programming is ideal for virtual instrumentation. Graphical icons replacing text commands are simply wire connected to each other by means of drag-and-drop operation. It simplifies the definition of measurement procedures by means of object-oriented data driven techniques.

The measurement procedure is obtained by drawing a procedural diagram or graph that defines data dependencies and operations to be performed. This block diagram is then translated into the computer executable code by some graphical programming compiler. The interpreter performs translation and immediate execution of the functional blocks without storing them in a file.

National Instrument's LabVIEW and Hewlett Packard 's HP VEE are the two popular graphical programming languages for virtual instrument applications. The use of VI integrates various measurement functions into a PC-based control environment that otherwise would have been accomplished by several stand-alone measurement instruments which are generally expensive or may not be available in a realistic application environment.